The present invention refers to a system for generating a voltage or current wave applicable to processes for the electrolytic colouring of aluminium, as well as to the system of autocontrolling same.
Electrolytic processes in general, and particularly processes for the electrolytic colouring, are faced with various limitations and difficulties of a diverse nature when an alternating current is used.
In this direction and in electrolytic processes, when two electrodes having a different nature are submerged in an electrolyte, the appearance of a continuous voltage therebetween is normal, which continuous voltage depends on the mentioned nature of said electrodes and on the composition of the electrolyte itself. If an alternating sine wave current is applied therebetween, the final result is that the previously mentioned polarization voltage is added to the alternate half-wave of the same sign and is deducted from that of the opposite sign causing an assymmetry, to a greater or lesser degree, with respect to the applied waveform.
More specifically, during processes for the electrolytic colouring of anodized aluminium, the layer of oxide which covers the metal presents two peculiar characteristics. Firstly, it is a very thin layer of oxide, that is--a nonconductor, which, when inserted between the metal and the electrolyte, acts as a condensor. Secondly, it has a greater facility for transporting electric charges from the metal to the electrolyte when the metal is negative, this facility being reduced when the metal is positive. This semi-conductor effect, together with the condenser effect, causes, when applying an alternating current, the positive half-wave with respect to the aluminium to present greater flow difficulties than the negative half-wave, giving rise in turn to drops in voltage differing from one direction to the other and, therefore, the waveform resulting from the applied voltage is not symmetrical; thus, there is a DC component to the applied electric signal, which is not always desirable. This is due to the semi-conductor effect. On the other hand, and due to the condensor effect, it is known that when an alternating current is applied between the aluminium and the other electrode, the condensor formed on the aluminium is charged to the peak voltage of the applied wave, the discharge being slower than the reduction in voltage due to the sine wave variation.
Thus, both the average value and the effectiveness of the resultant voltage are greater than those corresponding to the applied wave and, furthermore, they are variable in each case inasmuch as they are dependent upon the capacity of the anodic layer, the thickness thereof, the condition thereof, the process of obtaining same, etc.
This effect is particularly important in industry when thyristors are used to control the alternating current. In this case, due to the high capacity of the loads commonly used which can reach 5.times.10.sup.5 microfarads, the resultant waveform can reach an average value almost double that corresponding to the applied voltage and, as always, exclusively dependent on the conditions and characteristics of the layer of oxide.
Thus, for the same applied alternating voltage, the resultant voltage varies in dependence upon the variation of the electric characteristics of the load, and consequently, it is very difficult to control. In processes such as that of electrolytic colouring, wherein the electrical energy should be applied with a very precise dosage, the previously mentioned effect becomes a serious drawback and various attempts have been made to overcome same by indirect control systems, but without success.
On the other hand, the use of thyristors in industry to control alternating currents or conduction angle-rectified currents, frequently gives rise to serious problems of radiofrequency interferences which is very difficult to overcome, such interference being a result of the functioning of a thyristor when the applied voltage is other than zero.
The generating system of the invention overcomes all these difficulties, achieving a perfect control of the wave used in the process.